Aromatic carboxylic acids, like benzoic acid, phthalic acid, terephthalic acid, trimethyl benzoic acids, naphthalene dicarboxylic acids etc., are used widely as intermediates in the chemical industry and are usually prepared from the corresponding alkyl aromatic compounds by oxidation with air in the presence of liquid phase, homogeneous catalysts like cobalt acetate, manganese acetate etc. U.S. Pat. No. 2,833,816 issued to Mid Century Corporation in 1958 discloses the preparation of terephthalic acid by the oxidation of para-xylene by air in acetic acid solvent, at around 200° C. and 200 psig pressure, in the presence of homogeneous, liquid phase catalysts comprising of cobalt, manganese and bromine. U.S. Pat. Nos. 5,693,856, 3,562,318A; 5,760,288; 6,160,159; 4,329,493; 4,593,122; 4,827,025; 4,835,307; 5,087,741; 5,112,992; and EP 0 754,673 A disclose various modifications and improvements of the process of U.S. Pat. No. 2,833,816 for the manufacture of terephthalic and many other aromatic carboxylic acids. Comprehensive reviews of the oxidation of alkyl aromatic compounds to aromatic carboxylic acids are available in Suresh et al, Industrial Engineering Chemistry Research, volume 39, pages 3958–3997, year 2000 and W. Partenheimer, Catalysis Today, volume 23, pages 69–158, year 1995. Phthalic acid is manufactured by the aerial oxidation of ortho-xylene in the vapor phase over vanadia-based catalysts.
Prior art methods for the manufacture of aromatic carboxylic acids using homogeneous liquid phase processes suffer from several disadvantages. The homogeneous catalyst used is not easily separable from the products thereby limiting the reusability of the catalysts. Prior art methods also use corrosive bromide promoters requiring the use of expensive titanium steel thereby rendering the process itself more expensive. Another disadvantage of prior art methods is that acetic acid is oxidised to CO and carbon dioxide. It is therefore important to modify presently used homogeneous, liquid phase processes for the manufacture of aromatic carboxylic acids.
Some of the improvements and modifications that are contemplated include: (1) replacement of homogeneous liquid phase catalysts by solid heterogeneous catalysts; (2) replacement of corrosive bromide promoters by non-corrosive compounds; (3) elimination or reduction of the significant acetic acid oxidation to CO and carbon dioxide (5–10 wt. % of the carboxylic acid); and (4) lowering the concentration of intermediates which are difficult to remove from the final aromatic carboxylic acid product, in the reaction product.
4-carboxy benzaldehyde is a typical example of an intermediate which necessitates elaborate hydrogenation and recrystallisation procedures in order to manufacture purified terephthalic acid required for the polyester industry. It is believed that the reduction in the oxidation of acetic acid to carbon dioxide and CO can be achieved by the use of more efficient radical promoters thereby allowing oxidizer temperatures to be lowered without reducing reaction rates.
Replacement of homogeneous catalyst system by a heterogeneous, solid catalyst system in the production of high purity aromatic carboxylic acids is desirable to facilitate easy separation of the catalyst (i.e., metal ions) from the products and for reusability of the catalyst system.
Jacob et al in the journal Applied Catalysis A: General, volume 182, year 1999, pages 91–96 described the aerial oxidation of para-xylene over zeolite-encapsulated salen, saltin and salcyhexen complexes of cobalt or manganese in the absence of added halogen promoters and using tertiary butyl hydroperoxide, instead of bromide ions, as the initiator at low temperatures. While significant conversion levels of para-xylene (up to 50–60%) were attained the main product was para-toluic acid. The yields of terephthalic acid were negligible. It was claimed that the feasibility of using a solid, non-Br-containing catalyst in the absence of any solvent including acetic acid for the para-xylene oxidation to toluic acid, (which is the first stage in the oxidation of para-xylene to terephthalic acid) was established.
Chavan et al in the Journal of Molecular Catalysis A: Chemical, volume 161, year 2000, pages 49–64 observed the formation of oxo-bridged cobalt/manganese cluster complexes in the reaction mixture and these complexes were attributed as the actual catalysts for the production of aromatic carboxylic acids.
U.S. Pat. Nos. 5,849,652; 5,603,914; 5,489,424; 5,167,942, 5,767,320 and 5,932,773 disclose monomeric metal complexes encapsulated in molecular sieves. However, there is no reference to encapsulated oxo-bridged organometallic cluster complexes in prior art.
In the investigations leading to the present invention, it was found that when complexes of cobalt, manganese, nickel, zirconium or any of their combinations were supported or encapsulated or grafted in solid supports, the yields of the aromatic carboxylic acids were always low in accord with the findings of Jacob et al published earlier and mentioned herein above.